Through the years, there have been many modifications made to aromatic-containing polymers (e.g., styrenic block copolymers) to change and improve their properties. One such modification is to sulfonate the polymers. Such sulfonated polymers include those disclosed in U.S. Pat. No. 3,577,357; U.S. Pat. No. 3,642,953; U.S. Pat. No. 3,870,841; U.S. Pat. No. 6,110,616; U.S. Pat. No. 5,239,010; U.S. Pat. No. 5,516,831; U.S. Pat. No. 5,468,574; U.S. Pat. No. 7,169,850; U.S. Pat. No. 4,505,827; U.S. Pat. No. 4,492,785; and US Pub. App. 2007/0021569 among many other patents and patent applications. Once a polymer containing sulfonation-susceptible units is polymerized, and if desired, hydrogenated, it can be sulfonated using a sulfonation reagent. Although there are a number of known chemical reagents and routes that can be used to incorporate sulfonic acid groups into sulfonation-susceptible polymers, the difficulty of sulfonating polymers without gelation is widely appreciated in the art. Gelation of polymers can be caused by chemical gelation, physical gelation, or a combination thereof. Besides leading to polymer gelation, undesirable chemical cross-linking can also lead to polymer precipitation and/or intractability. Physical gelation, on the other hand, can be caused by non-covalent cross-linking. Physical gelation normally can be disrupted through appropriate solvent conditions. For example, Li, et al. Reactive & Functional Polymers, 56:189 (2003) describes the “insolubility” of sulfonated poly[styrene]-block-[2-[(per-fluorononenyl)oxy]ethyl methacrylate] in toluene as being due to “physically cross-linked network in the block copolymer resulting form the intermolecular associations of the ionic dipoles in the system.” It teaches that the addition of polar co-solvent readily enables the dissolution of the polymer.
The literature teaches the use of various acyl sulfates, which can be readily prepared from carboxylic acid anhydrides and sulfuric acid, for the sulfonation of aromatic-containing polymers without the formation of a significant amount of sulfone cross-linking groups. Although chemical gelation can be reduced or controlled by the use of acyl sulfates, physical gelation or polymer precipitation still poses a serious problem for polymer sulfonation. To reduce physical gelation or polymer precipitation, the reaction media of choice for the acyl sulfate methods disclosed in the literature are typically halogenated solvents such as dichloroethane. Halogenated solvents are alleged to not only afford solubility to the unsulfonated polymer and the acyl sulfate reagent (e.g., acetyl sulfate), but also to maintain the resulting sulfonated polymer in soluble form (e.g., a homogeneous liquid), without precipitation or disabling gelation. The use of halogenated solvent is, however, highly undesirable from an environmental, health, and safety standpoint. Methods that can effectively sulfonate aromatic-containing block copolymers in non-halogenated aliphatic solvents with equal or greater levels of sulfonic acid incorporation than in halogenated solvents would be highly desirable. Advantages of non-halogenated aliphatic solvents include, for example, (a) not suffering from the substantial environmental concerns associated with halogenated solvents; (b) typically being used in the preparation of the starting block copolymers, thereby enabling the sulfonation of polymer without the need for polymer isolation and re-dissolution prior to sulfonation; and/or (c) being suitable solvents for subsequent downstream processing of the sulfonated polymer into films, membranes, coatings, and the like. Lower acyl sulfates such as C2-C8 sulfates, especially C2 to C4 sulfates have many advantages over the higher acyl sulfates or other sulfonation reagents. These lower acyl sulfates, as with other acyl sulfates, are capable of sulfonating aromatic rings with negligible sulfone formation, thereby proceeding without substantial chemical gelation.
The process for sulfonating certain styrene containing block copolymers with lower C2 to C4 sulfates is taught in copending patent application Ser. No. 60/885,804, filed Jan. 19, 2007, having a common inventor. The polymer resulting from the process of the '804 application is disclosed and claimed in prior US Pub. App. 2007/0021569, also including common inventors.
However, when using C2 to C8 acyl sulfates as the sulfonation agent, residual carboxylic acids are formed as a byproduct. Residual carboxylic acids, such as isobutyric acid in the sulfonated polymer solution are difficult to remove from the resulting polymer composition when the solvent is removed to form an article or end use application. For example, the presence of such residual carboxylic acid is a significant problem when it is desired to form membranes from the sulfonated polymer by casting the polymer. Such residual acid remains in the membrane, and evaporates slowly over time leading to internal stresses. The internal stress leads to catastrophic failure of the cast membrane by forming cracks. In addition, the membranes cast from solutions containing such residual carboxylic acids smell of the residual acid, and the smell is offensive. What is needed is a solution to the serious problems when using C2 to C8 acyl sulfate as the sulfonation agent.
Therefore, there is still a need in the art for a method for producing sulfonated aromatic-containing polymers in non-halogenated aliphatic solvents that (1) is substantially free of polymer precipitation and disabling gelation; (2) can efficiently reach a high degree of sulfonation; (3) uses lower acyl sulfates such as isobutyryl sulfate as the sulfonation reagent; and (4) doesn't result in offensive odor and internal stress in any resulting membrane cast from the polymer composition.